Ultraviolet (UV) light is well known for its ability to destroy microorganisms. Certain wavelengths of UV light in the UV-C spectrum are very effective in destroying bacteria, viruses, mold spores, protozoa, etc. Many of these organisms can be airborne and are hazardous to human health. As such, UV light in the germicidal range is well suited for purifying air that humans breathe. Many air purification systems are on the international market that incorporate a UV light for this purpose.
UV light in the UV-C spectrum is not visible to the human eye. However, exposure to UV-C light even in small doses and for short periods of time is harmful to the health of human beings and animals. Specifically, eyes and skin will be damaged when exposed to UV-C light. This is also true to a lesser degree for exposure to the UV light in the UV-A and UV-B range. Furthermore, UV light negatively affects the integrity of many different materials, especially plastics. As such, UV compatibility of materials is an important part of the design and construction of any device that produces UV light. Furthermore, any device that incorporates UV light must be designed in such a way that no UV light can escape and endanger the health of humans and animals.
Some conventional UV air purification applications consist of a UV reaction chamber, which has high intensity UV light inside and a method to move the air to be purified through this reaction chamber. Typically, this is done with some sort of a fan, compressor, or in some cases, convection flow. The overall UV intensity within that UV reaction chamber determines how long the air must remain in this chamber in order to reduce harmful microbes that are present in the air to a desired level. For instance, in a very high UV intensity chamber, airborne viruses may require an exposure of 1 second for a 99.9% reduction. In a lower intensity environment, the exposure may require several seconds to achieve the same reduction.
A typical UV air purifier would include A UV reaction chamber through which the air flows. This chamber often times is cylindrical but can also be a cube or other shape. This chamber can be made of a material that is somewhat UV reflective, which will increase the overall UV dose. As an alternate, the UV chamber can be coated with a reflective material.
Inside the UV reaction chamber is a UV light source. Typically, this would be a low-pressure mercury vapor lamp designed to generate UV light in the germicidal range. These lamps can be made in a number of different shapes, but the most common are straight tubular lamps or variations in which the tube is twisted into different shapes. Such a design could be a U lamp where the tube is twisted into a U shape, or a spiral design where the tube is twisted into a spiral shape. UV reaction chambers also include a fan or other air moving mechanism that blows the air to be purified through the UV reaction chamber where it is exposed to UV light.
A design where air needs to flow through a UV reactor chamber presents a number of challenges. First, the air needs to flow freely through the chamber where it is being exposed to ultra-violet light. The chamber needs to be designed in such a way that there is very little restriction to airflow. Ideally, a chamber such as a cylindrical chamber would have to be opened on both ends so a relatively small fan can freely move the air through the cylinder at a relatively high flow rate.
Second, the chamber needs to be designed in such a way that no UV light exits the chamber in order to prevent damage to eyes, skin and construction materials such as plastics. Such a design would generally require a UV light barrier on the inlet and outlet side of the UV chamber. In order for the barrier to be effective, it would have to create a rather tortuous path for the light to travel before the light reaches the outside. In the tortuous path, the barrier would have to absorb the UV light to prevent it from exiting.
The above two criteria appear to work against each other. A design that creates a tortuous path for the UV light also creates a tortuous path for the airflow. As such, a good UV light barrier becomes a great restrictor for airflow requiring larger fans, which by their nature create more noise. For an application such as a residential air purifier, conventional methods have proven to be unsuitable because the large fan requirements for overcoming the high airflow restrictions create unacceptable noise levels.